An ultrasonic fuel-gauging system for an aircraft is described in WO 2005/015134. An ultrasonic transducer is attached to a carrier tape which is coupled to an internal surface of the wall of the fuel tank. A problem with such internally mounted systems is that the wires and transducer, being electrical components, can potentially ignite fuel. Another problem with such systems is that the tank must be entered by personnel in order to repair or inspect the transducer. This can be time consuming and difficult.
Both of these problems could be solved by externally mounting the transducer as in U.S. Pat. No. 5,761,955. However this is unacceptable in the case of an aircraft skin panel because the transducer will increase drag.